Various sensors are known in the pressure and temperature sensing arts. The ability to detect pressure and/or temperature is an advantage to any devices which are under variable temperature conditions, which can be severely affected by these conditions. An example of such a device is an automobile tire, which of course, experiences variations in both temperature and pressure. Many different techniques have been proposed for sensing the pressure and/or temperature in tires, and for delivering this information to the operator at a central location on the vehicle so that he knows that a tire is at low or high air pressure.
Such sensors generally communicate with the vehicle so that the sensed pressure and/or temperature are displayed to the operator when the vehicle is moving, i.e. the wheel rotating relative to the body of the vehicle. Such devices are generally relatively complex and expensive or alternatively are not particularly robust.
Some tire pressure and/or temperature sensor systems incorporate a sensor that is fixed to the body so no rotating electrical contact between the rotating wheel and the chassis is required. In this system, a sensor rod is deflected by contact with the tire sidewall when the sidewall of the tire is deformed as occurs when the tire pressure is low. This system provides an indication of low tire pressure but is not robust. For example mud or other debris on the wheels may cause faulty readings. Furthermore, this system provides an indication only when the tire pressure is reduced significantly as is necessary for significant tire bulge to occur. Clearly such a system simply cannot provide a reading of actual tire pressure.
In another form of fixed sensor the height of the vehicle can be detected and when the height is reduced, it is deemed tire pressure is low. However, if the tire in a rut or is parked on uneven ground, a faulty low-pressure reading is likely to be generated.
More complicated systems are capable of monitoring tire pressure. For example, some pressure sensor systems utilize a rotating encoder formed by a multi-polar ring of magnetic segments of different polarity that are distributed circumferentially in a regular and alternating manner. A transmitter coil coaxial with the ring and a fixed pickup (an induction coil system) is energized by alternating electrical current flowing through the transmitter coil to generate a magnetic field superimposed on the magnetic field created by the multi-polar ring generates a signal picked up and delivers a signal relating the rotating characteristic of the wheel and thus, the state of the tire.
Some tire pressure systems also utilize a wheel system wherein each sensor on each wheel is provided with a radio transmitter that transmit the information on tire pressure, etc. from the wheel to a radio receiver on the body of the vehicle and this transmitted signal is decoded to provide information on tire pressure etc. and makes it available to the operator. Conventional wireless systems, however, are not durable and are expensive to design and produce.
One type of sensor that has found wide use in pressure and temperature sensing applications, such as, vehicle tires, is the Surface Acoustic Wave (SAW) sensors, which can be composed of a sense element on a base and pressure transducer sensor diaphragm that is part of the cover. For a SAW sensor to function properly, the sensor diaphragm should generally be located in intimate contact with the sense element at all pressure levels and temperatures.
One of the problems with current SAW sensor designs, particularly those designs adapted to tire pressure and temperature sensing applications, is the inability of conventional SAW sensing systems to meet the rigorous environment within the tire itself. Such systems are inherently expensive, awkward, and often are not reliable in accurately sensing tire air pressure and temperature. It is therefore believed that a solution to such problems involves a low cost SAW sensor packaging system, which can be integrated into a tire and interrogated wirelessly. Such a system is described in greater detail herein.